It is well known to those skilled in the art that low density rigid polyurethane and polyisocyanurate foams can be prepared by reacting and foaming a mixture of ingredients including an organic polyisocyanate (including diisocyanates) and an appropriate amount of polyol or mixture of polyols in the presence of a volatile liquid blowing agent, which is caused to vaporize by the heat liberated during the reaction of polyisocyanate and polyol. It is also well known that this reaction and foaming process require the use of amine and/or metal carboxylate catalysts as well as surfactants. The catalysts ensure adequate curing of the foam while the surfactants regulate and control cell size. The terms polyisocyanurate foams and polyurethane modified polyisocyanurate foams describe the same general class of rigid foams and are used interchangeably in the industry.
In the class of foams known as low density rigid polyurethane or polyisocyanurate foams, the blowing agent of choice has been trichlorofluoromethane (known in the art as CFC-11). These types of foams are closed-cell foams in which the CFC-11 vapor is encapsulated or trapped in the matrix of closed cells. They offer excellent thermal insulation due in part to the very low thermal conductivity of CFC-11 vapor and are used widely in insulation applications such as roofing systems, building panels, refrigerators, and freezers. Generally, about 1 to 60 parts by weight, and more specifically, about 15 to 40 parts by weight of blowing agent per 100 parts by weight polyol are used in rigid polyurethane or polyisocyanurate formulations.
Chlorofluorocarbons (known as CFCs) including CFC-11 are now suspected ozone depleting compounds which also contribute to the greenhouse warming effect in the atmosphere. Other chlorofluorocarbons suspected of possessing similar detrimental effects on the earth's atmosphere include dichlorodifluoromethane (known in the art as CFC-12) and 1,1,2-trichloro-1,2,2-trifluoroethane (known in the art as CFC-113). Based on the MONTREAL PROTOCOL ON SUBSTANCES THAT DEPLETE THE OZONE LAYER and the CLEAN AIR ACT, alternatives to CFC solvents, propellants, refrigerants, and blowing agents are being developed and commercialized rapidly.
Hydrochlorofluorocarbons (known as HCFCs) are viewed as acceptable alternatives to CFCs because they are inherently less chemically stable in the earth's atmosphere, and have lower ozone depletion potentials and greenhouse warming potentials than fully halogenated CFCs.
Certain hydrochlorofluorocarbons are known to be viable commercial alternatives to the chlorofluorocarbon blowing agent, CFC-11, currently being employed in the production of rigid polyurethane replacements for CFC-11 are 11-dichloro-1-fluoroethane (known in the art as HCFC-141b) and 1,1-dichloro-2,2,2 -trifluoroethane (known in the art as HCFC-123) because they both possess key physical properties similar to CFC-11 including boiling point and thermal conductivity.
To ensure successful commercialization of these alternatives, it is necessary that they be chemically stable under required processing conditions and end uses. In particular, HCFCs are well known to undergo degradation by dehydrohalogenation reactions to form halogenated alkenes. Examples of such reactions are as follows: ##STR1##
Many of these haloalkene products possess unknown properties and it is therefore desirable to hold their formation to a minimum as a precautionary measure.
Tests performed using the above hydrohalocarbons as blowing agents, in typical foam formulations now in commercial use, revealed that the haloalkenes can be found in the cells of the cured foam at concentrations up to about 10,000 parts/weight per million relative to the blowing agent.
Stabilizers have been added to hydrohalocarbons to inhibit or minimize the generation and buildup of degradation products. For example, U.S. Pat. No. 4,861,926 teaches that 1,1,1-trichloroethane can be stabilized with mixtures of epoxybutane, nitromethanes, 2-methylfuran, and methyl acetate in textile dry cleaning and metal degreasing applications. Kokai Patent Publication 103,843 published May 22, 1986 teaches that the addition of benzotriazole stabilizes 1,2-dichloro-1-fluoroethane when it is exposed to metallic surfaces in the presence of hydroxylic solvents, e.g. water or alcohols. The abstract of Japanese 2,204,424 published Aug. 14, 1990 teaches that hydrochlorofluoropropanes in the presence of steel are thermally stabilized by adding nitro compounds, phenols, amines, ethers, esters, epoxides, alcohols, ketones, or triazoles.
Specialized chemical additives are often present in low density rigid polyurethane and polyisocyanurate foams to enhance certain performance features of the foam e.g. flame retardants, antioxidants, and solubilizing surfactants. Such additives are dissolved in a formulation component or pre-mix prior to foam production. Flame retardants include halocarbons, e.g. chloroalkyl phosphate esters, polybromoalkanes, or polybromoaromatics. Antioxidants are typically phosphite esters. Solubilizing agents commonly used are ethoxylated nonylphenols.
We considered the use of additives to lower the concentration of volatile haloalkene degradation products generated in rigid polyurethane and polyurethane modified polyisocyanurate foam formulations blown with saturated hydrohalocarbons.